Resonant circuit and radiator



March 29, 1949. N. w. ARAM RESONANT cmcun AND RADIATOR 5 Sheets'-Shet 1Filed Oct. 2, 1945 NUUDOQ INVENTOR. NATHAN W ARAM HIS ATTORNEY March 29,1949. N, ARA 2,465,416

' RESONANT CIRCUIT AND RADIATOR Filed Oct. 2, 1945 5 Sheets-Sheet 2SOURCE OF P Tf/VT/fl BEAT FREQUENCY UTILIZATION CIRCUIT FIG. 8

INVENTOR. NATHAN W ARAM HIS ATTORNEY March 29, 1949.

Filed 001;. 2, 1945 N. w. ARAM RESONANT CIRCUIT AND RADIATOR 5Sheets-Sheet 3 FIG. l2

FIG. ll

INVENTOR NATHAN w. ARAM HIS ATTORNEY March 29, 1949. N. w. ARA 2,465,416

I RESONANT CIRCUIT AND RADIATOR Filed 001:. 2, 1945 5 Sheets-Sheet 4INVENTOR NATHAN -w. ARAM SOURCE QF W'M' @2561; HIS ATTORNEY March 29,1949. N. w. ARAM 2,465,416

RESONANT CIRCUIT AND RADI ATOR I Filed Oct. 2, 1945 5 Sheets-Sheet 5FIG. 15

INVENTOR NATHAN W. ARAM WMQ h w HIS ATTORNEY Patented Mar. 29 1949UNITED STATS ATENT OFFlCE Nathan W. Aram, Chicago, Ill., assignor toZenith Radio Corporation, a corporation of Illinois Application October2, 1943, Serial No. 504,717

6 Claims. 1

This invention relates to electric translating systems and moreparticularly to such systems which include a resonant cavity.

High frequency radio signals, Whose wave length is of the order of fivemeters or less, may be conveniently translated through resonantcavities. Any suitable cavity havin conductive boundaries is spaceresonant. Such cavities are usually made cylindrical. toroidal orrectangular and are usually resonantin complex modes and at manyfrequencies.

It is an object of my invention to provide a new and improved electrictranslating system comprising a cavity resonant in a simple mode, thatis, one in which resonance of a single type is established in a harmonicseries of frequencies. In this type of cavity, the resonant frequency isa linear function of wavelength. the larger the cavity, the greater isthe wavelength.

It is also an object of my invention to provide a new and improvedelectric translating system which is space resonant in such a way that areenforcement of energy substantially at a focal. point within the spaceis produced,

It is a corollary object of my invention to plO- vide such a new andimproved electric translating system in which external circuits may becoupled to the system at such a focal point.

Still another object of my invention is to provide such a new andimproved energy radiator which radiates energy into space in desireddirections.

It is a further object of my invention to provide a new and improvedresonant cavity electric translating device which may be readilydesigned with simple parameters so that, upon construction. it isresonant at a predetermined frequency, and its resonance issubstantially of a single type in a harmonic series of frequencies.

It is an additional object of my invention to provide a new and improvedspace resonant translating device especially suitable for excitation bya wide variety of exciting means.

It is still another object of my invention to provide a new and improvedform of spaceresonant cavity having an input and an output and onecapable of providing highly efficient filtering action between suchinput and output.

The features of my invention which I believe to be novel, are set forthwith particularity in the appended claims. My invention itself, both asto its organization and manner of operation, together with furtherobjects and advantages thereof may best be understood by reference tothe following description taken in connection with the accompanyingdrawings in which:

Figure 1 illustrates one embodiment of my in vention;

Figure 2 illustrates schematically a principle of operation of myinvention;

Figures 3 through 9 illustrate alternative embodiments of my invention;

Figure 10 illustrates schematically a principle of operation of theembodiment of Figure 9;

Figures 11 through 15 illustrate still other embodiments of myinvention; and

Figure 16 illustrates certain characteristics of an embodiment of Figure15.

In Fig. 1 a space-resonant cavity is formed by a pair of conductiveconfocal paraboloidal shells l3 and H with their concave sides facingeach other. The two shells l0 and H are clamped together at their edgesby rings l2 and is held together by bolts i l. The depth of each of theparaboloidal shells ill and H, measured from a plane passing between thetwo shells perpendicular to their common axis to the deepest point ofeach shell, is equal to the focal distance of the paraboloid definingthe shell. That is, the focus of each one of the paraboloidal shells H)and H coincides with the focus of the other shell.

Energy is transferred from a suitable source into the cavity bounded bythe paraboloidal shells to and it through a coaxial transmission lineincludin an inner conductor and an outer cylindrical concentricconductor 16, the terminals of the source [5 being connected to theinner and outer conductors. The coaxial line including the innerconductor and outer conductor 56 is terminated by a small loop H at thecommon focus of the two paraboloidal shells H] and H, said inner andouter conductors each being connected to opposite terminals of loop l1.

Energy may be abstracted from the space bounded by the shells to and ll, through a second coaxial transmission line including an innerconductor l8 and an outer conductor l9, terminated at any point withinthe shell by a small loop 20.

In Fig. 2 there is illustrated graphically the manner in which spaceresonance in a simple mode is produced between the two paraboloidalshells H3 and H, these shells being identical With those illustrated inFigure l and being given like reference characters. The point 2 l, whichis the confocal point at which the loop ll of Figure 1 is placed, is thefocus for each of the parabolas H3 and i i. It is a property of aparabola that a line drawn from its focus to any point along theparabola intersects the parabola at the same angle as the parabola iscut at the same point by another line parallel to the axis of theparabola.

A ray 22 of energy is illustrated in Figure 2 proceeding from the focalpoint 2! to impinge upon parabola H0 at a point 23. By Well known lawsof reflection this ray 22 is reflected at the point 23 from parabola illin a direction illustrated by line 24 parallel to the axis of parabolaill; the angle 25 of incidence and the angle 26 of reflection 'beingequal.

The two axes of the two parabolas l0 and I I, as

pointed out previously in connection with the paraboloidal shells i8 andi l of Figure 1, are common to each other so that the ray 24 is parallelnot only to the axis of the parabola it but also to the axis of theparabola ll. Since the axes and focal points 2i of the parabolas It andII coincide, the ray 24, when it strikes parabola l I, is reflected as aray 2'5 toward the focal point 2|, the angle 23 of incidence and angle29 of reflection being equal.

This process of double reflection and return to the focus takes placefor any energy ray emitted from the focus H. A ray 3% emitted from thefocus 2| in a different direction from the ray 22 is reflected fromparabola it as a ray 3% parallel to the axes of the parabolas it and Hand is reflected from parabola H as a ray 32, which passes through thefocus 2!. No matter in what direction a ray is emitted from the focus 25, after being reflected from each of the parabolas l G and H insuccession, it returns to the focus 2 i.

The length of path traveled by a ray of energy emitted in any directionfrom the focus 2i is always the same when it first returns to the focus2|. That is, the length of path traveled by a ray of energy followinglines 22, 2d and El in a single circuit from the focus 2! and back tothe focus 2! is the same as the length of path traveled by a ray ofenergy in a single circuit along the lines 30, 3| and 32.

It may be proved mathematically that such paths, from focus 2! through areflection from each of the p'arabolas Id and i I back to the focus 2|,are always of the same length. If it be assumed that the parabola iii bedefined by the equation:

y =4ax (1) then the focus 2i is at a distance a from the point onparabola Ill at which it is cut by its axis. The perpendicular distancefrom any oint :2, y, on parabola iii to a line through focus 2iperpendicular to the axis of the parabola id is or, expressed in termsof y,

The distance from focus 2! to the point x, y (the coordinates of point2! being a, is

or expressed in terms of y:

t o-e The sum of these two distances expressed by Equations 3 and 5,from the point a, 0 to point 3:, y, and from point :r, y to a linethrough a, 0, perpendicular to the axis of parabola Ill is:

the axis of parabola It may be simplified, and is then found to be:

This mathematical analysis indicates that the distance along path 22 andhalf of the length of line 24 is equal to 2a, or that the distance alongpaths 22, 24 and 27 is equal to 4a. In other words, a ray of energyemitted from focus 2| in any direction travels away from focus 2|, isreflected once from each of the parabolas Ill and il, and returns tofocus 2| in phase after having traveled a distance equal to twice thedistance between the parabolas i9 and H measured along their commonaxis. Therefore, all rays travel from focus 2! and are reflected oncefrom parabola H3 and once from parabola H, and return to focus 2| inphase.

The small loop I! is effective in transferring energy between the end oftransmission line I6 and the space surrounding loop ll through themagnetic component of electromagnetic radiation in the vicinity of loopIT. A dipole may be substituted for loop I! so that energy istransferred through the electric component of the radiation near thedipole.

For any particular size of parabolas I0 and II and with loop IT or withthe substituted dipole, resonance occurs at a harmonic series offrequencies uniquely determined by the reflection path length. When sucha resonant condition exists, it is called resonance of a single type.

The loop I! is made as small as possible with respect to the dimensionsof the cavity enclosed by shells l0 and N. This is preferable in orderthat energy radiated from loop ll shall appear to come as nearly from apoint source as is possible. Since the loop i"! must necessarily havefinite size, energy of frequencies within a finite band of frequenciescentered about a mean fre-- quency determined by the ideal point sourceresonator produces resonance within the space bounded by shells iii andH. This finite band of frequencies has a width measured in frequencywhich width is a function of the size of loop i! with respect to thesize of the space bounded by shells It and ii. That is, as loop I? ismade smaller, the finite band of frequencies within which resonanceoccurs is smaller.

In order to make the relative size of loop H small, so as to obtain acavity bounded by shells It and H resonant at a very narrow range offrequency, it may be desirable to make that space fundamentally resonantat a much lower frequency which frequency is a sub-harmonic of thedesired frequency. It may be desirable to make line 96 an integralnumber of half waves in length, in order that impedance of the line I 6shall not substantially affect the resonant frequency of the spacebounded by shells it and H,

The phenomenon of space resonance by alternate concentration anddispersion of oscillatory energy from a focal point in a space may beutilined in several very useful ways. In Fig. 3 a pair of paraboloidalshells at and ii have their concave sides facing each other and arespaced apart with their deepest points, measured along their commonaxis, at a distance apart equal to twice the focal distance of eitherone of the paraboloidal shells 41s or :3 l The two shells are identicaland their focal distances are correspondingly identical. These twoshells it and H are identical with shells i9 and i l of Fig. 3. exceptthat they do not meet at their rims, so that a 32 is left open all ofthe way around the two shells it and i i. Proper spacing between the twoshells it and 31 is maintained by supporting members :23, which arepreferably formed of such material and so spaced that they provideminimum interference with energy radiated from within the shells it] and45 to external space.

Radiation of such energy from within the shellsdi! and ll to externalspace is M d M indicated by ar rows an a.

Oscillatory energy at a frequency such that space resonance is producedwithin the shells 46 and M is transferred from a generator, not shown,to the common focus of the shells Gil and 4! through a coaxialtransmission line including an external cylindrical conductor l5, and aconcentric inner conductor M. This transmission line comprisingconductors 46 and El is terminated at the focus of the shells ill and Hin a short dipole d8, of which one arm is connected to inner conductorll and the other arm to the outer conductor 46. The length of the armsof dipole 48 may, but need not, be an integral fractional part ormultiple of the wave length of the energy radiated into shells 50 and4!.

Space resonance is produced between the shells 40 and ll by reason ofthe fact that energy radiated from dipole 48 in directions interceptingeither of the shells ii) and M is reflected once from each of the shellsail and Ill and returned to dipole =28. Radiation from between theshells All and 4! passes out through the gap 22, not only by reason ofradiation of energy directly from the dipole 48 but also by reason ofenergy reflected back to dipole .8 from shells it and M, such reflectedenergy being thereafter radiated by the dipole 48 through gap 42.

Energy thus radiated from the gap (l2 between the shells i-ll and ll isconcentrated in directions from the dipole 58 through the gap 12. If thegap 42 be mad quite small, this concentration of energy can be madegreat. With the structure shown, in which the gap 12 extends completelyaround the shells 45 and ll, energy is radiated substantially in aplane. If one of the shells (ill or H be cut off more than the othershell, so that the gap 62 faces up or down, energy is correspondinglyradiated in such upward or downward direction.

Paraboloidal shells, such as shells Hi and ii in Figure l, or shells 45and ii in Figure 3, are separated by twice their focal distance measuredalong their common axis, since they must have a common focus, and, asexplained above, this separation bears a direct relation to the resonantfrequency of the space enclosed by the shells, so that the configurationof the paraboloidal shells, which is related to their focal distance,bears a definite relationship to the resonant frequency of the spaceenclosed thereby. It has been found in practice, however, that it is notdifficult to shape such paraboloidal shells and space them apart aproper distance so as to be resonant at a desired frequency.

The necessary dimensional accuracy to which shells if} and H mustconform is greater as the resonant frequency approaches more closely asingle desired frequency as distinct from a band of frequencies. Somemechanical distortion of of one or both shells it and H may be effectedto alter over a narrow range the resonant frequency of the space boundedby those shells.

Th dipole 48 shown in Figure 3 should have an overall length which is avery small part of a Wave length in order that it shall appearsubstantially as a point source of radiant energy within the spacebounded by shells ill and ll. The mechanism of such radiation is similarto that explained in connection with the loop ii in Figure 1.

It is desirable that the surge impedance of the transmission linecomprising conductors it and 4'! be substantially matched to theimpedance of dipole 43 coupled to the space bounded by shells 40 and 41.

As with the loop H, the smaller the dipole 48" with the devices ofFigures 1 and 3. The edges ofshells 5ft and 5% are joined at all pointsexcept for a small opening 52 at one point along the perimeter.Oscillatory energy represented by arrow 53 escapes in substantially asingle direction through this opening 52. The device in this formierefore affords transmission of oscillatory en-- orgy in a highlyconcentrated beam.

Two truncated cones 55 and 55 provide excitation for the space withinshells 53 and 5|. Both cones are truncated near their apices, and havethe truncated portions facing each other with the base portionscentrally affixed respectively in shells 5b and 59. The common axis ofthe two cones 5t and 55 coincides with the common axis of the shells 5ftand 5E. The cone 55 may desirably be made hollow, but is entirely closedWithin th shell 56. The cone 55 is hollow and the truncated end affordsan opening 56 adjacent the closed truncated end of the cone 5 3.

Shells 50 and 5 I, as well as cones 54 and 55, are made of reflectingmaterial so that they bound the space which is enclosed by them. Theymay, for example, be made of conducting material, such as metal, or ofmaterial having a high dielectric constant. A wave guide or conductivepipe 5'! is connected with the base portion of the cone 55, where it isjoined to the shell 5|, and oscillatory energy within the wave guide 51is transferred through the cone 55 and out through opening 56 into thespace enclosed by shells 50 and 5d. The radiator, or cone 55, whichnarrows down to the small opening 55, provides that oscillatory energyfrom the wave guide 51 is radiated within the space enclosed by shells50 and 51 substantially only at the common focus of the shells 55 and5E.

The size of the opening 55 is, as stated, small with respect to the sizeof shells 59 and 5!, but it is also sufliciently large with respect tothe wave length of the wave passing therethrough so that undesirablylarge attenuation does not occur. Similarly, the size of the wave guideor pipe 57 is sufficiently large with respect to the wave length of thewave transmitted therethrough that attenuation of the wave in passingthrough the pipe is not intolerably large.

When energy is transferred between pipe 51 and the space bounded byshells 50 and 5|, resonance of a single type occurs, as previouslyexplained in connection with the previous forms of the invention.

In Figure 5 certain parts of the arrangement are similar to parts of thearrangement shown in Figure 4, and have like reference numerals. Asomewhat different form of excitation is provided, the structure inFigure 4 being excited through a wave guide while the structure inFigure 5 is excited from a spark or are gap. In Figure 5 the spark orarc gap is formed by a pair of cones 60 and t l, whose apices arerounded, the cones being placed with their rounded apices adjacent sothat, upon the application of suitably high potentials between the conesB0 and 6!, sparks or arcs pass therebetween. Any space current flow maybe induced between cones 60 and BI so long as there are high frequencycom- 1 ponents of that current of frequency comparable tothe fundamentalfrequency of the resonant cavity formed by shells i! and 55.

The cones 69 and 6! are preferably formed of a metal, such as copper,aluminum or silver, having low resistance to the flow of heat. Each coneis preferably solid and is integrally formed with a conductor ofsubstantial cross-section. Conductor 62 is joined with cone 5t andconductor 63 is joined with cone 5 8. The conductors 52 and 63 each havea set of flanges or fins numbered respectively 64 and st, for the betterdissipation of heat flowing from the cone out through the conductor.

Each cone is insulated from the conductive shells 50 and 5! by aninsulating support. That is, cone 6!) is fastened to a hollow truncatedconical section 65 by a truncated conical insulating portion 51, theconical section 66 being joined at its base to the shell 59. Similarly,the cone BI is joined to a conical section as by a conical piece ofinsulation 6%, the conical section 68 being joined to the shell 5i atits base.

Since the insulating sections t'l and ts isolate the shells 50 and 5!from the cones 8t and 6 l, the shells may be grounded and suitable highpotentials may be applied to conductors t2 and 63 for the formation ofsparks or arcs between cones 60 and 6|. When sparks or arcs containingenergy of certain wavelengths are so formed, such high frequency energyis radiated from between the two cones 60 and ti into the space boundedby the shells 50 and 5|, so that energy of those certain wavelengthscomparable to the physical size and configuration of the shells 5t andti is selectively radiated through the hole 52, as illustrated by thearrow 53.

All forms of the invention illustrated in Figures 1 through 5 have beenconstructed of that conic section termed a parabola, and are formed ofcircular paraboloi-dal surfaces. Other conic sections may be utilized toform surfaces of revolution bounding a space which is resonant in amanner useful in practicing my invention. In fact, other conicoids orsecond degree tridimensional surfaces may be used, such as confocalhollow conicoids.

'In Figure 6, a hollow sphere it, with a small loop H at its center fedby a coaxial transmission line including outer conductor l2 and innerconductor 13 encloses a space resonant in a single mode when excited byenergy of a certain frequency transmitted through the line into thatspace. Energy transmitted through the line to the loop "H and radiatedwithin the sphere i0 is reflected from the surface of the sphere 10directly back to the loop H.

If it be desired that the hollow spheroid to, loop H, and transmissionline including conductors 12 and 13 act as a two-terminal resonantnet-work of Very sharp resonance, the loop H should be as small aspossible in comparison with the dimensions of the sphere iii in orderthat the loop shall appear as a point source. On the other hand, if itbe desired that the spheroid 10, loop H, and the line act as atwo-terminal band pass filter, the loop H may be made larger to obtainthe desired band pass characteristics or the shell in be made adistorted spheroid. In fact, in all modifications of my invention, theexciting source within the resonant space may be made of such dimensionwith respect to the dimensions of that space so as to cause the space toact as a band pass filter of desired band pass characteristics.

'If desired energy may be extracted from the space bounded by shell litby providing a loop ex tending into that space as shown for example byloop 28 in Fig. 1.

If it be desired, a dipole may be substituted for the loop H. The dipoleis connected to its line in the same manner as dipole 48 in Fig. 3 isconnected to lines 46, ii.

In Fig. 7 another method of excitation is shown for a structure similarin certain respects to the structures illustrated in Figures 1 through5. A pair of:v paraboloidal conducting shells 8d and M are arranged withtheir concavities facing each other and with common focal points. shells8D and 8| are completely reflecting except in two opposite directionswhere openings are left for the egress of radiation. These openings aresealed hermetically respectively by covers 62 and 83, which are made ofa material substantially transparent to the desired radiation and whichmay be formed in such a lens shape that radiation tends to pass from thespace bounded by the shells 8B and Si in substantially parallel lines,as indicated by the dotted lines tit and 85.

Means is provided at the common focal point of the two shells and forcreating corona of high intensity in the region of that common focalpoint. As illustrated, two conducting members tit and 87 are placedclose to the common focal point on either side of it with a gap 88therebetween within which the corona may be formed when suitableionizing potential is impressed between the conducting members 36 and87. Means including a suitable source of potention 89 and conductors 90,passing respectively to the two conducting members 86 and 6? through ashield 91, are provided for the purpose of impressing suitable ionizingpotential between the members 36 and 31. The shield s: is so arrangedinternally as to provide a hermetic seal through which the conductors 9tpass, so that the space bounded by the shells at and iii is sealedhermetically.

An atmosphere is provided in the space between the shells 8B and 35 ofsuch nature and at such pressure that the potential supplied from source89 between arms 86 and 3? produces corona of a desired intensity in thegap 88. The formation of corona is attended by the production of highfrequency potentials of substantial intensity. Such high frequencycorona potentials existing between the members 86 and iii areaccompanied by radiation of energy over a wide band of frequencies inthe space bounded by shells 3d and 8!, which radiation includes aparticular frequency, of which the corresponding wavelength iscomparable as has been heretofore explained in connection with thestructures illustrated in Figs. 1 through 5 to the dimensions of thespace bounded between shells 8i and iii. Energy of such fre-- quency bydirect paths and reflected paths passes out through lenses 8?. and 83 asbeams. By suitable choice of the character of the gaseous atmospherefilling the space between shells 8t and 3| and by suitable correlationof the pressure of that atmosphere with the potential supplied fromsource 89, the intensit of the potential supplied from source 89, theintensity of the radiation coming through lenses Bland 83 may be mademaximum.

The physical size of the discharge produced in the gap'88 and theoverall length of the members 86 and 81 of the corona producing meansinfluences the band width of frequencies which may be emitted. general,the larger the sizes the greater the bandwidth of frequenciesemitted-and These two the smaller is the selectivity produced by thecomposite shells 80 and SI. That is, when it is desired to make thespace broadly resonant, so that the band width is relatively large, themembers 85 and 3'! may be made like a dipole, as illustrated, but themembers 86 and 87 are made as small as possible when sharp resonance orhigh selectivity is desired.

In Fig. 8 a modified form of space resonant device, bounded by surfacesof revolution formed of conic sections, is shown. This device isfundamentally resonant at substantially two different frequencies.Energy is transferred into the device through a coaxial transmissionline Iilfl, terminated by a small loop ILI. Space on one side of a planepassing through the center of the loop iill is bounded by a pair ofparaboloidal shells H32 and IE3, the two shells having their concavesides facing each other and having a common focal point at the center ofthe loop IilI. Space on the other side of the plane passing through loopillI is bounded by two additional paraboloidal shells I84 and I05 havingtheir concave sides facing each other, having a common focal point atthe center of the loop Ill! coinciding with the common focal point ofthe shells I02 and H33, and each having a major axis of different lengthfrom the major axes of shells I02 and Hi3. The pair of shells I94, I 05and similarly, the pair of shells I82, I93 are similar to halves ofshells 5D and 5! in Figure 5. If desired, the space between the edges ofshells IE2 and I03 and the edges of shells HM. and H25 may be closed b asuitably shaped strip I66 lying in the previously mentioned planepassing through loop IOI.

Energy transferred through the coaxial line Hill and to the loop IilI isradiated into the space bounded by the doubly resonant chambercomprising shells I92 and I133. The chamber comprising shells IBZ andI93 is resonant at certain frequencies corresponding to the dimensionsof that space, in a manner explained in connection with the structuresof Figs. 1 through 5. Similarly, due to the shape of the chambercomprising shells H34 and I05, en-

ergy of a wavelength comparable with the shell size, such energy beingtransferred through line I08 and radiated from loop MM, is selectivelyreenforced, the wavelength and the corresponding frequency of thatenergy being determined by the dimensions of the space bounded by shellsIM and I05. Space resonance is therefore produced in the space boundedby shells I52, I93, I94 and I65 at substantially two discretefundamental frequencies, and the impedance appearing between the twoconductors of the coaxial line I06 is correspondingly affected at thesetwo discrete frequencies.

Energy may be abstracted from the space bounded by shells I82 and IE3through a small loop IIQ connected to a coaxial transmission line IIl'I.Such energy abstracted through the loop Iiil and line It? ispredominantly of the frequency determined by the space resonance of thespace bounded by shells I92 and IE3.

Similarly, energy of a frequency determined by the dimensions of a spacebounded by shells I94 and I85 may be abstracted from that space by asmall loop I68 and a connected coaxial transmission line Hi9.

When a loop and connected line such as the loops Hi3 and led and linesIt)? and I09 are utilized, the structure may be used as a filter,resonant at two discrete frequencies, for separating out energy at oneof those frequencies. A1-

it) ternatively,-energy may be fed into lines Ill! and m9 atrespectively different frequencies and such energy transferred throughthe space bounded by shells Iili, I83, iil l, and I to line IElIl,where, for example, the energy of two different frequencies may be mixedby heterodyne action.

Such heterodyne action takes place when line set is connected across acrystal detector I33 in series with a condenser i3 3. Condenser I34 haslow reactance at the frequencies of energy in lines iiil and 5% and highreactance at their beat frequency. Beat frequency voltage appearingacross condenser I3 may be utilized in device I35.

In Fig. 9 there is shown another structure in which still another conicsection is utilized to form a surface of revolution bounding a spacewhich is resonant at a single predetermined frequency. In this case theconic section is an ellipse rotated about its major to form the desiredsurface of revolution. That is, the space is bounded by the shell of aprolate spheroid. The shell ii s is formed of a reflecting material inthe shape of the resulting surface of revolution. Excitation of thespace bounded by shell I I9 may be provided through. a small loop III atone of the foci of the ellipse determining the shape of shell IE9 andthrough another small loop H2 at the other focus. The small loop IIIterminates a coaxial transmission line II3, of which the outer conductoris connected to the shell H9 and the inner conductor is connected to oneterminal of the secondary winding of a high frequency transformer l M.The small loop I I2 terminates another coaxial transmission line II5, ofwhich the outer conductor is connected to shell I It and the innerconductor is connected to the other terminal of the secondary winding oftransformer Ill. The anodes H6 and Ill of a high freonency electrondischarge device H8 are connected in push-pull relation to respectiveterminals of the primary winding of transformer II l.

Ultra gh frequency energy of a wavelength comparable to that determinedby the dimens ons of ell se H9, is generated in the device H8 oramplified through it, and is then transferred through transformer IM tothe small loops III and M52 in bush-pull, or balanced, relation, that isin opposite phase. The size of shell I I9 is such that energy of thefrequency of the wave from device H8, radiated from loop HI andtraveling by refl ction f o t e shell I69 to the loop H2, reaches theloop I I2 in such phase as to establish a r wual resonance between thetwo foci. This resonance is of a single type at a fundamental frequencyor harmonic.

In Fig. 10 there is a graphical illustration of the path taken by suchradiated and reflected energy. The shell II9 is represented by anellipse I28 and a single ray of radiation I2I is represented astraveling from the focal point I'M occupied by loop Ill in Fig.9 It is awellknown proposition (see Coordinate Geometry by Fine and Thompson,published by The McMillan Company in New York in 1937, on page 84) thatthe tangent at any point of an ellipse makes equal angles with the linesjoining the points to the foci of the ellipse. According to thisproposition, therefore, the ray I2 I, after reflection from the ellipseI223, takes the path E22 to the other focus E23 of the ellipse.

It is also a known property of an ellipse that the sum of the lengths ofthe paths III and I22 between the first focus IM and the second focus 23is 'a constant, no matter in what "direction thepath I 2I is oriented.

It follows from the above-mentioned geometrical properties of theellipse I20 that all energy radiated from focus I2 l, as along therepresentative path I25, must arrive at focus I23 after one reflectionfrom the ellipse I29 in'predetermined phase relation.

If the path. I22 be extended through focus I22,'as illustrated by pathI25, so as to impinge again upon the ellipse I22, energy is reflectedfrom the ellipse I25} along a path I26 passing through the first focusI24, the sum of the lengths of paths I25 and I 26 being equal to the sumof the length of paths I2! and I22. Similarly path I26 may be extendedthrough focus I2-t as math I21, and after reflection from the ellipseI20 as path I28, energy traveling along those paths passes back throughthe second focus I23 again. Extension of the path I28 through focus I23, as path 129 results in still another reflection from the ellipseI2Il, as illustrated by path I311, which extends again back to theoriginal focus I2 3.

Following the energy paths I2I, I22, I25, I25, I21, I28, I29 and I36 inorder, it is evident that energy radiated from either focus passesthrough the other focus and then. alternately through the first focusand the other focus many times, and, after each reflection, approachesmore and more nearly a path directly through the two'foci'l23 and I24.It is therefore evident that energy'radiated from the foci 523 and I2tends to become concentrated along the major-axis of the ellipse. Sincethe prolate spheroidal shell I I9 in Fig.9 is formed by revolution ofthe ellipse i293 about its major axis, the same action as illustrated inFig. 10 takes place in any plane passing through the major axis.

Because of the peculiar property of an ellipsoid, formed by revolutionof an ellipse about its major axis, in reflecting radiation. inside sothat it ultimately is concentrated along the major axis, such anellipsoid arranged as a space resonant cavity is useful in radiating anarrow concentrated beam of energy. The ellipsoidal shell H9 in Fig. 9has a hole I3I at oneendin. line with the major axis and in line withthe two small loops .III and H2. Direct and concentrated radiationenergy from the loops I II and H2. is radiated as a narrow con--centrated beam of energy through the hole I3I, the beambeingrepresentedby an -arrow I32.

.'In Fig. 11 an ellipsoidal shell I4; encloses a space into which energyis radiated from a dipole IM, energy being abstracted from thespacethrough a dipole I42. The dipole MI is energized througha coaxialcable M3, of which the inner-conductor is connected to one arm of thedipole I4! and the outer conductor of the.

line I43 to the other arm. The dipole M2 is similarly connected to acoaxial cable I44 through which energy within the space bounded by theshell I40 isabstracted.

.A representative path for energy traveling from .the:.dipole IM to.the=dipole 142 is ;illus tratedras aline I45 from the dipole MI to arepresentative .point onthe'shell Mill, and another line-I 45,representing the path taken by reflection of the ray traveling the pathI i-5 and striking the representative point, the ray It's passing--eventually .to the dipole I422. The ray I46, easit 'passesthe dipoleI42, proceeds in a path I4I.to.strikeanotherpoint of the shell I48 andbe reflected along a'fourth path 143 back to the dipole i ii.

In Fig. 12 a shell Mil has'a small loop IF-Il at one focus and anothersmall loop 'I5I at the other focus, those two small loops being.connected respectively, as are the dipoles M5 and M2 in ll, to thecoaxial transmission lines Hi3 and Hi l. In operation, the structureshown in Fig. 12 is similar to the operation described for the structureof Fig. 11. That is, energy radiated into the shell I m from loop lfi isabstracted by loop E52.

The structures of Figs. 11 and 12 may be used as filters for selectingfrom a wave having harmonics, desired ones of the harmonics. In thatcase, the physical dimensions of the dipoles I ll and I i-2 and loopsHit and 556 as related to th'e size of shells determine the band passcharacteristics of the filter. When employed as a filter, energy in aband of frequencies'related to the size of the components of theresonant cavity is radiated from dipole MI or-loop I fiil and thenselectively transferred to the other dipole or loop Iti.

In 13 there is illustrated a conicoid, par ticularly an ellipsoid, whichis not a surface of revolution and in which the three dimensions are alldifferent. For clearness, the shell I82 is illustrated viewed from threediiferent'directions, the relation between those directions beingindicated respectively by dotted lines 56!, N32, and iiiil. That is, thedotted lines lEI illustrate that the length of shell 'Ifili viewed fromthe top and from the front elevation is the same, lines H32 illustratethat the height of the shell viewed from the front elevation or from oneend is the and lines I53 illustrate that the thickness of shell i533viewed from the top 'or from the end is the same.

A small loop the is placed substantially at one focus of the shell itsand a second small loop I555 is placed substantially at the other focus.It is to be understood that shell l te does not have true foci but thedimensions transverse to the long dimension do not diiler too greatly,the loci of sections of shell 5% taken through that long dimensionnearly coincide and may fall Within the compass of loops 56 i and Such acondition is tolerable in a band pass filter. These two loops Hit andI65 are connected respectively to a source of high frequency wave and aload (not shown).

Any plane passing through the two foci, at which the loops I64 and I65are located, cuts the shell its in an ellipse. Consequently radiationfrom one of the loops I65 or I65 in that plane is resonant at afrequency corresponding to the size of the ellipse cut by such plane.Another plane cutting the shell I63 at a different angle and passingthrough the loops I54 and H35 cuts the shell Itll in an ellipse of adifferent size in which resonance of a different frequency is produced.

:As illustrated in Figure 13, a vertical plane through the loops i6 3and I65 cuts'the largest ellipse possible from the shell I (it! and ahorizontal plane through the-loops I84 and I55 cuts the-smallest ellipsein which resonance maybe produced. These two sectionscorrespond'respectively to the lowest and the highest frequency at whichresonance is produced .in the space bounded by the shell I60, andresonance is produced at all intermediate frequenciesibyintermediatesections cut by planes passing :through the loops I64 and I65. Thisstructure1istherer fore a band-pass filter in which the cut-offfrequencies correspond respectively to the highest and lowestfrequencies described, corresponding respectively to the smallest andthe largest. ellipses in which space resonance is produced.

In general any conicoid may be excited in accordance with my inventionso that in at least a single plane, space resonance is produced. Thatis, so that in at least that one plane energy returns from alldirections in predetermined fixed phase relation.

In Fig. 14 a radiator I10, similar to the radiator illustrated in Fig.3, except that the shells 40 and II are closed for a major part of theircircumference, is energized through a concentric transmission line I'IIfrom a source I11 of pulsed wave energy at a suitable frequency andradiates pulsed short bursts of high frequency electromagnetic energyrepresented by arrows I12 upward in a mountain pass encompassed betweentwo mountain sides E13 and I14.

A plane, or other object, I 15, passing above the mountain pass betweenmountains I13 and I14, intercepts the pulsed rediation I12 and reflectsit back towards the radiator I10, as indicated by arrow I15. Thereflection detector I11, of known form, deflects such reflected pulsedradiation and, if desired, measures the time elapsed between itstransmission from radiator I and'its later reception thereby afterreflection, thereby indicating not only the presence of plane I15 butits altitude or distance from radiator I10.

In Fig. 15 there is illustrated still another resonant cavity bounded bysurfaces of revolution formed by conic sections rotated about theiraxes. A hyperboloidal shell I80 is placed with its concave side facingan ellipsoidal shell I 8|, with a respective .foci coincident at theposition of a small loop I82 connected to a concentric transmission lineI83 arranged to transmit energy into the space bounded by the shells I80and I8I. Energy radiated from the loop I82 toward the shell I99 isreflected from shell I80 as though it were radiant from the conjugatefocus of the hyperbola defining shell I80. perbola has a focus at theloop I82, and it has also a conjugate focus behind the shell I80 fromwhich such energy after reflection from the shell I80 appears to beradiant. That radiant reflected energy from shell I80, upon impingingupon shell ISI and reflection therefrom is directed back toward the loopI82.

Energy so reflected successively once from shell I so and once fromshell IBI and so returning to loop traverses a circuit which is the samelength for energy radiated from loop I82 in all directions. Hence, suchenergy returns to loop I82 in the same phase and resonance isestablished at a harmonic series of frequencies determined by the pathlength.

In Fig. 16 a graphic illustration is used to demonstrate why energyradiated in any direction from loop IE2 is, after one reflection fromeach of shells I80 and I0: of Fig. 15, returned to loop I82 inpredetermined phase relation. In this figure the X axis is representedby a line I99 and the Y axis by a line IIII, and a hyperbole,symmetrical about both axes, is illustrated as having two arms I92 andI93. Revolution of arm I92 about axis I9I is effective to define theshell I80 of Fig. 15. The hyperbola having the two arms I92 and I99 hasrespective focal points I94 and I95. A line I98 drawn from the focus I94to a representative point I91 along the arm of I92 of That is, the hythehyperbole represents a ray of energy radiant from loop I82 in Fig. 15.

An ellipse having any desired eccentricity is constructed about the twofoci I99 and I95, and a portion I of this ellipse, forming with a partof the arm I92 of the hyperbola a closed space containing the line I99,is illustrated.

It is a known property of an hyperbola that the tangent at any pointthereof bisects the angle included by the lines extending from eachfocus to that point (see page 198 of the previously identified text onCoordinate Geometry). The tangent I99 at point I91 on the arm I92 of thehypcrbola in Fig. 16 therefore bisects the angle A between the line I99and the line 200 joining the focus I with the point I91. Extension ofthe line 299 throughpoint I91 to a point 20I on the ellipse I93 forms anangle B between this extension 292 and the tangent I99, such angle beingequal to the angle C between line 200 and the lower part of the tangentI99, which in turn is half the angle A between the line 200 and the lineI96. In consequence, the angle D between line I95 and the tangent I99 isequal to the angle B between the line and the tangent I99. The line 292therefore represents also the path of a ray which traveled toward thearm I92 along the path I99 and which was reflected from the arm I92 ofthe hyperbola at point I91.

It has been previously explained in connection with Fig. 16 thatlines200-292 and 293, joining the foci I95 and I99 respectively to point 2!,form equal angles with the tangent to the ellipse I99 at point ZI'iI,because the points I94 and I95 at the ends of those lines are the fociof the ellipse. Consequently, the path 299 is the path of a rayreflected from a point 2% of ellipse I98 after traveling along path 202to the ellipse. Therefore, a ray emitted from focus I94 in any directionalong a representative path I99 is reflected from the arm I92 of thehyperbola along path 202 and is then reflected from the ellipse I93along path back to the focus I 99. Conversely. a ray radiated from thepoint I94 along a representative path 293 takes the opposite circuit.

It is also demonstrated below that the total length of the path fromfocus I94 along the line I99 and then along lines 202 and 203 back tothe focus I99 is always the same, no matter in what angular directionthe rays first leave the point I9 2. It may be shown that the distancesof any point along an arm of a hyperbola from the foci differ by aconstant distance (see page 104 of the text on Coordinate Geometry citedpreviously). Futhermore, as explained in connection with the ellipseillustrated in Fig. 16, the sum of the distances from any point along anellipse to its foci is a constant distance. Therefore (the numbersplaced within brackets representing the length of the designated linesin Fig. 16 for convenience in forming equations) expressing well knownproperties of an ellipse and an hyperbola:

(200) minus (196)=k1 (200) plus (202) plus (203) =k2 Subtracting thefirst of these equations from the second:

(196) plus (202) plus (203) =kz-k1 It is therefore demonstrated that thepaths I96, 202 and 203 in Fig. 16 represent the true re flection path ofa ray radiated from focus We in any direction and the total length ofthe three reflection paths is always the same no matter in whatdirection the ray be originally radiated.

Therefore, energy radiated from loop 92 in. Fig. 15 returns in phase tothe loop after one complete circuit, having started from loop in anydirection, so that the space hounded. I. theshells i539 and 101 of Fig.15 is space r onant in the same fashion as is the case with structurespreviously described.

It is evident from inspection of the various structures which I haveillustrated that o con'icoids or second degree tridin" I may be utilizedas surfaces noun spaces which resonance of a single type is established.Such resonators are useful for many purposes besides those I havedescribed, and they are in general useiul at ultra high frequencies forthe same purposes for which lumped tuned circuits are useful at lowerfrequencies.

While I have shown and described the particular embodiments of myinvention, will be ohvious to those skilled in the art that changes and.modifications may he made Without departing from my invention in itsbroader aspects, and I, therefore, aim in the appended claim to cove allsuch changes and modifications as l wit e true spirit and scope of myinvention.

I claim:

1. A high quality singly resonant cavity comprising a pair ofsubstantially identical circular paraboloidal reflecting shells having acommon focus, the concavities of said shells being juxtaposed incontinuous relation to each other, thereby defining a substantiallycontinuous inner space, and means for radiating energy within said spaceat said common focus whereby radiated energy after reflection from eachof said shells returns to said focus in predetermined phase relation.

A high quality singly resonant cavity comprising a pair of substantiallyidentical curved conductive surfaces having a common focus, each of saidsurfaces being defined by revolution of a parabola about its major axis,said surfaces being substantially coaxially disposed with theconcavities thereof juxtaposed in contiguous relation to each other, andmeans for radiating ergy at said common focus whereby radiated energyafter reflection from each of said surfaces returns to said focus inpredetermined phase relation.

3. A high quality singly resonant cavity comprising a pair ofsubstantially identical confocal paraboloidal reflecting shells havingtheir concavities juxtaposed in contiguous relation to other, an openingin said cavity communicating with outer space, and means for radiatingenergy in said cavity at a point such that the radiant energy afterreflection from each of said shells returns to said point inpredetermined phase relation and energy is radiated through said openinginto said outer space.

lien a 4. In combination, a pair of substantially identical circularparaboloidal reflecting shells having a common focus, said shells beingdisposed with the concave sides facing each other, and corona dischargemeans for radiating energy at said common focus, whereby radiant energyafter reflection from each of said shells returns to said focus inpredetermined phase relation.

5. In combination, a pair or substantially identical curved conductivesurfaces having a common focus, each of said surfaces being defined byrevolution of a parabola about its major axis, said surfaces beingsubstantially coaxially disposed with the concavities thereof juxtaposedin contiguous relation to each other, and corona discharge means forradiating energy at said common focus, whereby radiant energy afterreflection from each of said surfaces returns to said common focus inpredetermined phase relation.

6. In combination, a pair of substantially identical confocal circularparaboloidal reflecting shells, the concave sides of said shells loeingjuio taposed in coaxial and contiguous relation to each other therebydefining a reflecting cavity, an opening said cavity communicating withan outer space, a cover hermetically sealing said opening, and coronadischarge means for radiating energy within said cavity at a point suchthat the radiant energy after reflection from each of said shellsreturns to said point in predetermined phase relation, said cover beingsubstantially transparent to said radiant energy, whereby energy isradiated through said cover into outer space in a predetermineddirection.

NATHAN W. ARAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 775,840 Bose Mar. 29, 1904781,606 Hewitt Jan. 31, 1905 1,278,026 Salto Sept. 3, 1918 1,304,868Franklin May 27', 1919 1,735,377 Caughlan Oct. 19, 1927 2,118,419Scharlau Sept. 15, 1932 2,044,413 Weyrioh June 16, 1936 2,106,770Southworth et a]. Feb. 1, 1938 2,129,713 Southworth Sept. 13, 19382,241,119 Dallenbach May 6, 1941 2,245,669 I-Iollmann June 17, 19412,250,934 0111 July 29, 1941 2,265,796 Boersch Dec. 9, 1941 2,281,274Dallenbach et a1. Apr. 28, 1942 2,281,550 Barrow May 5, 1942 2,354,658Barber Aug. 1, 1944 2,445,784 Lehmann July 27, 1948

